📚 Explaining Pressure | 解释压强
Pressure is a central idea in CIE AS-Level Physics. It connects forces, fluids, and gases, and it appears in topics from mechanics to thermal physics. This article explains how pressure arises, how to calculate it, and how to interpret pressure in solids, liquids, and gases.
压强是 CIE AS 物理中的一个核心概念。它把力、流体和气体联系起来，出现在力学到热物理等多个主题中。本文将解释压强如何产生、如何计算，以及如何理解固体、液体和气体中的压强。
1. Defining Pressure | 压强的定义
Pressure is defined as the force acting normally per unit area on a surface. In symbols:
压强定义为作用在表面单位面积上的正压力。用符号表示：
p = F / A
a in square metres (m²). The SI unit of pressure is the pascal (Pa), where 1 Pa = 1 N m⁻².
这里 F 是法向力，单位牛顿 (N)；A 是面积，单位平方米 (m²)。压强的 SI 单位是帕斯卡 (Pa)，1 Pa = 1 N m⁻²。
In calculations, the force must be perpendicular to the surface. If a force acts at an angle, only its normal component contributes to pressure.
在计算中，力必须垂直于表面。如果力与表面成一定角度，只有它的法向分量才会产生压强。
2. Scalar Nature and Units | 压强的标量性质与单位
Pressure is a scalar quantity, even though it comes from a force vector. It has magnitude but no specific direction at a point in a fluid; it acts equally in all directions at that point. In solids, the pressure on a surface is usually associated with the normal direction, but the quantity itself is still scalar.
压强是标量，尽管它来源于矢量力。在流体中某一点，压强有大小但没有特定方向；它在该点向各个方向作用相等。在固体中，表面上的压强通常与法线方向有关，但物理量本身仍是标量。
Common units include the pascal (Pa), kilopascal (kPa), atmosphere (atm), millimetre of mercury (mmHg), and bar. For CIE questions, always convert to Pa unless a ratio is required.
常用单位包括帕斯卡 (Pa)、千帕 (kPa)、大气压 (atm)、毫米汞柱 (mmHg) 和巴 (bar)。对 CIE 题目，除非要求比值，否则一般先换算成 Pa。
- 1 atm = 1.01 × 10⁵ Pa
- 1 bar = 1.00 × 10⁵ Pa
- 1 mmHg ≈ 133 Pa
3. Pressure Exerted by Solids | 固体产生的压强
urface is horizontal. For example, a block of mass m has pressure p = mg / A on the floor.
静止在表面上的固体产生的压强等于其重量除以接触面积，前提是重量竖直向下且表面水平。例如，质量为 m 的木块对地面的压强为 p = mg / A。
If the same block is turned onto a smaller face, the force remains the same but the area decreases, so the pressure increases. This explains why sharp blades and pointed nails cut or penetrate more easily.
如果把同一木块转到较小的面上，力不变但面积减小，因此压强增大。这就解释了为什么锋利的刀刃和尖钉更容易切割或穿透物体。
Exam tip: When calculating pressure from a solid, identify the contact area carefully; it is the actual area touching the surface, not the total surface area of the object.
考试提示：计算固体压强时，要仔细确认接触面积；它是实际接触表面的面积，而不是物体的总表面积。
4. Pressure in Fluids at Rest | 静止流体中的压强
In a fluid at rest, pressure at a point depends on the depth below the free surface, the fluid density, and the gravitational field strength. It is independent of the shape of the container.
在静止流体中，某一点的压强取决于它到自由液面的深度、流体密度和重力场强度。它与容器的形状无关。
A fluid exerts pressure on any surface in contact with it. The pressure acts perpendicular to that surface. At the same horizontal level in the same fluid, the pressure is equal, provided the fluid is continuous and at rest.
流体会对与其接触的任何表面施加压强。压强垂直于该表面。在同一流体的同一水平面上，只要流体连续且静止，压强相等。
For a liquid exposed to the atmosphere, the total pressure at depth h is:
对于暴露在大气中的液体，深度 h 处的总压强为：
p = p₀ + hρg
where p₀ is the pressure at the surface, usually atmospheric pressure.
其中 p₀ 是液面处的压强，通常为大气压。
5. Deriving p = hρg | 推导 p = hρg
the column is V = Ah, so its mass is m = ρV = ρAh. The weight of the column is mg = ρAhg.
考虑一段高度为 h、横截面积为 A 的竖直液柱。液柱体积为 V = Ah，因此质量 m = ρV = ρAh。液柱重量为 mg = ρAhg。
he base area A gives the pressure difference between the bottom and the top of the column:
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